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Arbitron is a consumer research company in the United States that collects listener data on radio audiences.

There are 2 ways that radio ratings are surveyed:

Arbitron's syndicated radio ratings service collects data by selecting a random sample of a population throughout the U.S., primarily in 294 metropolitan areas, using a paper diary service two to four times a year and Portable People Meter (PPM) electronic audience measurement service 365 days a year.

Responding to requests from its customers — radio broadcasters, ad agencies and advertisers — that expressed their interest in the collection of more accurate ratings data, Arbitron introduced the Portable People Meter (PPM) service in 2007.

The PPM is a wearable portable device much like a pager or cell phone, that electronically gathers inaudible codes that identify the source of a broadcast, such as a radio station. Arbitron recruits and compensates a cross section of consumers to wear the meter for an average of one year and up to two years. The audience estimates generated from each monthly survey is used as the buy/sell currency for radio stations and advertisers/agencies

http://en.wikipedia.org/wiki/Arbitron